Replace changing string including uppercase character with lowercase character

[silfer]

Hello

My input lines :

<p>Material blabla ;<lb/>Dimension blabla ;<lb/></p>
<p>Sideral blublu ;<lb/>Sticky Gonzalez blibli;<lb/>Reductio ad absurdum bloblo</p>

For each "<lb/>" followed by an uppercase character, I want it replaced with a white space + same character but lowercase.

My wished output :

<p>material blabla ; dimension blabla ;<lb/></p>
<p>Sideral blublu ; sticky Gonzalez blibli; reductio ad absurdum bloblo</p>

I made several re.sub trials with unsuccessful regular expressions. I also read several things about it : it seems not so easy for a beginner in programming like me. So, if you have not an easy solution, please tell me what I should learn first before understanding it.

Thank you